Shorten diagnostic and refer to bug report instead

diag::warning_module_shadowing_may_break_module_interface was *enormous* but still described the problem too tersely to convey everything that was needed. Instead, reference a bug report that describes the problem and its workarounds in more detail.

Author: beccadax

include/swift/AST/DiagnosticsFrontend.def

@@ -351,12 +351,10 @@ ERROR(error_opening_explicit_module_file,none,
 ERROR(error_extracting_flags_from_module_interface,none,
       "error extracting flags from module interface", ())
 WARNING(warning_module_shadowing_may_break_module_interface,none,
-        "public %0 %1 shadows module %2, which is permitted by Swift but may "
-        "stop other compiler versions from importing module %3 or its clients; "
-        "please work around this language bug (SR-898) by renaming either the "
-        "%0 %1 or the module %2, or by passing the '-Xfrontend "
-        "-module-interface-preserve-types-as-written' flags to the Swift "
-        "compiler",
+        "public %0 %1 shadows module %2, which may cause failures when "
+        "importing %3 or its clients in some configurations; please rename "
+        "either the %0 %1 or the module %2, or see "
+        "https://bugs.swift.org/browse/SR-14195 for workarounds",
         (DescriptiveDeclKind, FullyQualified<Type>,
         /*shadowedModule=*/ModuleDecl *, /*interfaceModule*/ModuleDecl *))
 REMARK(rebuilding_module_from_interface,none,

test/ModuleInterface/module_shadowing.swift

@@ -24,14 +24,14 @@
 
 #if !SELF_SHADOW
 import ShadowyHorror
-// OTHER-DAG: ShadowyHorror.module_shadowing:{{[0-9]+:[0-9]+}}: warning: public class 'ShadowyHorror.module_shadowing' shadows module 'module_shadowing', which is permitted by Swift but may stop other compiler versions from importing module 'module_shadowing' or its clients; please work around this language bug (SR-898) by renaming either the class 'ShadowyHorror.module_shadowing' or the module 'module_shadowing', or by passing the '-Xfrontend -module-interface-preserve-types-as-written' flags to the Swift compiler{{$}}
+// OTHER-DAG: ShadowyHorror.module_shadowing:{{[0-9]+:[0-9]+}}: warning: public class 'ShadowyHorror.module_shadowing' shadows module 'module_shadowing', which may cause failures when importing 'module_shadowing' or its clients in some configurations; please rename either the class 'ShadowyHorror.module_shadowing' or the module 'module_shadowing', or see https://bugs.swift.org/browse/SR-14195 for workarounds{{$}}
 
 @_implementationOnly import TestModule
 #endif
 
 public struct ShadowyHorror {
-  // OTHER-DAG: module_shadowing.swift:[[@LINE-1]]:15: warning: public struct 'module_shadowing.ShadowyHorror' shadows module 'ShadowyHorror', which is permitted by Swift but may stop other compiler versions from importing module 'module_shadowing' or its clients; please work around this language bug (SR-898) by renaming either the struct 'module_shadowing.ShadowyHorror' or the module 'ShadowyHorror', or by passing the '-Xfrontend -module-interface-preserve-types-as-written' flags to the Swift compiler{{$}}
-  // SELF: module_shadowing.swift:[[@LINE-2]]:15: warning: public struct 'ShadowyHorror.ShadowyHorror' shadows module 'ShadowyHorror', which is permitted by Swift but may stop other compiler versions from importing module 'ShadowyHorror' or its clients; please work around this language bug (SR-898) by renaming either the struct 'ShadowyHorror.ShadowyHorror' or the module 'ShadowyHorror', or by passing the '-Xfrontend -module-interface-preserve-types-as-written' flags to the Swift compiler{{$}}
+  // OTHER-DAG: module_shadowing.swift:[[@LINE-1]]:15: warning: public struct 'module_shadowing.ShadowyHorror' shadows module 'ShadowyHorror', which may cause failures when importing 'module_shadowing' or its clients in some configurations; please rename either the struct 'module_shadowing.ShadowyHorror' or the module 'ShadowyHorror', or see https://bugs.swift.org/browse/SR-14195 for workarounds{{$}}
+  // SELF: module_shadowing.swift:[[@LINE-2]]:15: warning: public struct 'ShadowyHorror.ShadowyHorror' shadows module 'ShadowyHorror', which may cause failures when importing 'ShadowyHorror' or its clients in some configurations; please rename either the struct 'ShadowyHorror.ShadowyHorror' or the module 'ShadowyHorror', or see https://bugs.swift.org/browse/SR-14195 for workarounds{{$}}
 
   init() {}
   public var property: ShadowyHorror { ShadowyHorror() }